Switch assembly having wafers of metal oxide varistor material

ABSTRACT

A plurality of elongated conductive contact element support members and a plurality of wafers of metal oxide varistor material are interleaved. Each of the wafers of metal oxide varistor material performs the dual function of insulating the adjacent conductive members contacting the opposed surfaces thereof as long as normal voltages appear across the members and of providing a progressively lower impedance between the conductive members when voltages across the members tend progressively to exceed normal voltages thereby minimizing arcing across the contact elements when used in inductive circuits.

United States Patent [191 Harden, Jr.

[54] SWITCH ASSEMBLY HAVING WAFERS OF METAL OXIDE VARISTOR MATERIAL [75] Inventor: John D. Harnden, jri, Schenectady,

[73] Assignee: General Electric Company [22] Filed: Dec. 22, 1971 [21] Appl. No.: 210,998

[52] US. Cl. ..200/166 C -[51] Int. Cl. ..H0lh 1/02 [58] Field of Search ..200/166 C, 166 J; 335/129, 335/127 [56] References Cited UNITED STATES PATENTS 3,166,661 111965 Yasui et al. ..200/166 J 3,259,717 7/1966 Amann ..200/166 I X 1 Jan.9, 1973 3,076,880 2/1963 Ehrismann ..335/129 3,345,592 10/1967 Kuke ..335/129 3,319,033 5/1967 Claesson et al ..200/166'J Primary ExaminerI-I. 0. Jones Attorney-John F. Ahern et air [5 7] ABSTRACT A plurality of elongated conductive contact element support members and a plurality of wafers of metal oxide varistor material are interleaved. Each of the wafers of metal oxide varistor material performs the dual function of insulating the adjacent conductive members contacting the opposed surfaces thereof as long as normal voltages appear across the members and of providing a progressively lower impedance between the conductive members when voltages across the members tend progressively to exceed normal voltages thereby minimizing arcing across the contact elements when used in inductive circuits.

6 Claims, 5 Drawing Figures SWITCH ASSEMBLY HAVING WAFERS OF METAL OXIDE VARISTOR MATERIAL The present invention relates in general to switches and in particular to switch assembly of the kind which includes a plurality of contact support conductive members which are stacked in insulated relationship with one another and are deflectable to produce engagement and disengagement of contacts located thereon.

Such switch assemblies are commonly used to control electrical motors and the like in response to mechanical, electromechanical, thermal and other inputs to the switch assemblies. ln the operation of such switch assemblies, the contacts undergo repeated and rapid'cycling to effect a making and breaking of electrical circuit connections. Such action produces sparking in the contacts which not only reduces the life of the contacts but also produces electromagnetic radiation which affects electromagnetic radiation ,response apparatus such as radios, television sets and the like. To overcome such effects, in the prior art, various impedance elements have been connected in shunt with the contact elements to reduce the sparking and hence prolong the life of the contacts as well as reduce the electromagnetic radiation produced thereby. Such elements include capacitor, resistors, and the like which represent additional elements which must be associated with the switch assembly and are limited in their effectiveness in performing the desired functions of reducing the arcing or sparking.

The present invention is directed to the provisions of new elements as well as a superior organization or integration of such elements into the switch assemblies which overcome the disadvantages inherent in prior art switch assemblies.

Accordingly, an object of the present invention is to provide improvements in switch assemblies.

Another object of the present invention is to provide a switch assembly in which particular elements are provided which reduce the number of elements required in the switch to perform the switching function as well as to prolong the life thereof and reduce the electromagnetic radiation produced by the use thereof.

Another object of the present invention is to provide a switch assembly of superior performance.

In carrying out the present invention, in one illustrative embodiment thereof, there is provided a pair of elongated conductive support members at least one of which is resilient and each of which have a pair of 'major opposed surfaces. Thebmembers are closely spaced with the surfaces substantially parallel. A pair of contact elements is provided, each contact element conductively secured to a respective support member. An element of metal oxide varistor material having a pair of opposed surfaces is provided. Each of the conductive support members are mounted to and supported by a respective opposed surface of the element of metal oxide varistor material. The conductive contact support members are aligned so that the contact element is engaged and disengaged in response to movement of the resilient conductive support member. The material has an alpha in excess of in the current density range of 10' to 10 amperes per square centimeter. The element is proportioned so that the standby current flow between conductive members in contact with spaced surfaces of the element is low when normal operating voltage appears across the conductive members and when voltages in excess of the normal voltage progressively appear thereacross rapidly decreasing impedance is presented by the element in accordance with the alpha of the body of material thereby limiting the variation of the voltage between the conductive members.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a front view of a switch assembly embodying the present invention,

FIG. 2 is a plan view of the embodiment of FIG. 1,

FIG. 3 is a sectional view of the switch assembly of FIG. 1 taken along section lines 33 of FlG. 1,

FIG. 4 is a front view of the switch assembly of FIG. 1 showing the switch assembly in an energized position,

FIG. 5 shows graphs of the electrical characteristics of three materials of different voltage gradients and alphas, suitable for use in the switch assemblies of the present invention,

Reference is now made to FIGS. 1, 2, 3 and 4 which .show a frame member 10 having a horizontally extending portion 11 and a vertically downward extending portion 12. An electromagnet 13 is provided having a winding 14 which terminates in a pair of terminals 15 and 16 and a longitudinal magnetic core 17 which is supported at one end in portion 12 of the frame member and the other end of which extends for the length of the horizontal portion 12. Also, provided is an assembly 20 a plurality of elongated conductive contact support members 21, 22, 23 and 24, each having a pair of opposed major surfaces. The contact support members are closely spaced with the surfaces substantially parallel. All of the members are resilient, permitting deflection thereof in a manner to be explained below. The conductive members 21-24 are provided with respective contact elements 25-28. The other ends of the conductive members 21-24 are provided with respective terminal elements 29-32. A plurality of insulating wafers 33, 34, 35, 36 and 37, each having a pair of opposed surfaces, is also provided. The wafers 33-37 are interleaved with the conductive support members 21-24 and a pair of clamping members 38 and 39 are provided at the ends of the stack array of conductive members and wafer members. Wafers 34 and 36 are constituted of metal oxide varistor material in accordance with the present invention and wafers 33, 35 and 37 are constituted of conventional insulating material. Wafers 33, 35 and 37 may also be constituted of metal oxide varistor material, if desired. Each of the conductive support members 21-24 are mounted to and supported by a respective opposed surface of the insulating wafers 33-37. The wafers 33-37 and the conductive members 21-24, as well as the end support clamping members 38 and 39, are provided with a plurality of aligned holes. Bolts 40 are inserted into the holes to fixedly secure the conductive members and the wafer members in place in insulated spaced relationship to the frame 10. The inner two conductive members 22 and 23 of the stack are limited in their movement. The element 23 is limited in its upward movement by the tap on insulating retainer 45 and the conductive element 22 is limited in its downward movement by another tab on the retainer 45. The conductive members 21 and 22 form a switch unit having a set of normal open contacts and the conductive members 23 and 24 form a switch unit having a normally closed set of contact elements. Movement or deflection of the conductive members 21 and 24 is effected by means of post member 46 located adjacent the contact elements 25-28. The post member 46 is secured in insulated relationship to the upper conductive member 24 extends downward through apertures in the conductive member 23 and conductive member 22 and is conductively secured to the lower conductive member 21. The lower end of the post member 46 rests on an insulating support on a horizontal leg of the rocker member 50 of magnetic material, including a horizontal leg 51 and a vertical leg 52. The junction of the legs of the rocker member is supported on a fulcrurn 53 formed in the frame member. The fulcrum 53 is in the form of a pair of side by side projections. The rocker member 50 is secured in pivotal relationship on the fulcrum by means of screw element 54 which is supported in the frame member 10.

Located side by side with the assembly 20 of conductive contact support members 21-24 is another assembly 20' of conductive contact support members identically supported as can be seen particularly in FIG. 2. Elements of the assembly 20 identical to elements of the assembly 20 are indicated by the samenumerals with primes.

The operation of the switch assembly in FIG. 1 can be readily perceived from FIG. 4 which shows the assembly in its energized condition. The application of voltage between the coil terminals and 16 causes the rocker member 50 to be pivoted clockwise with the horizontal leg 51 moving the post 46 upward to close normally open contacts 25 and 26 to open normally closed contacts 27 and 28. In the operation of such a relay for motor control and for the operation of other electrical transducers, sparking occurs because inductive elements are included in the circuits of the motors and transducers. Upon disengagement of the contacts, inductive currents continue to flow and develop high voltages across the opened contacts which produces the sparking to'maintain the current flow.

ln accordance with the presentinvention, utilization of insulating wafers of metal oxide varistor wafers in place of conventional insulating wafers provides not only insulating and mechanical support function but also the function of the limiting of the voltage across the contact support members. Arcing is reduced and spurious electromagnetic radiation is substantially reduced. Thus longer contact life, options in choice of contact materials for properties other than erosion resistance, reduction of contact support arm heating are achieved. Such results are achieved without the need for additional elements. The wafers of metal oxide varistor material may be substituted for the conventional insulating wafer elements without need for making special mechanical provisions therefor. The particular metal oxide varistor material utilized and the proportioning thereof depends on the voltage versus current characteristics desired.

The wafers 34 and 36 are constituted of metal oxide varistor material such as described in Canadian US. Pat. No. 831,691, which has a nonlinear voltage versus current characteristic. The metal oxide varistor material described in the aforementioned patent is constituted of fine particles of zinc oxide with certain additives which have been pressed and sintered at high temperatures to provide a composite body or wafer of material. The current versus voltage characteristic of the composite body is expressed by the following equation:

where Vis voltage applied across a pair of opposed surfaces or planes, I is the current which flows between the surfaces, I C is a constant which is a function of the physical dimensions of the body as well as its composition and the process used in making it, i

a is a constant for a given range of current and is a measure of the nonlinearity of the current versus voltage characteristic of the body.

In equation (l), when V is used to denote voltage between opposed planes of a unit volume of material, or voltage gradient, current flow through the unit volume of material in response to the voltage gradient becomes current density. For the metal oxide varistor material for current densities which are very low, for example, in the vicinity of a microampere per square centimeter, the alpha (or) is relatively low, i.e., less than 10. in the current density range of from l0 to l0 amperes per square centimeter, the alpha is high, i.e., substantially greater than 10 and relatively constant. In the current density ranges progressively in excess of 10 amperes per square centimeter, the alpha progressively decreases. When the current versus voltage characteristic is plotted on log-log coordinates, the alpha is represented by the reciprocal of the slope of the graph in which current density is represented by the abscissa and voltage gradient is represented by the ordinate of the graph. For a central range of current densities of from 10" to l() amperes per square centimeter, the reciprocal of the slope is relatively constant. For current densities below this range, the reciprocal of the slope of the graph progressively decreases. Also for current densities above this range, the reciprocal of the slope of the graph progressively decreases.

The voltage gradient versus current density characteristics of three types of material in log-log coordinates are set forth in FIG. 3. Graphs 60 and 61 are materials of high voltage gradient material and graph 62 is a graph of low voltage gradient material. For all of the graphs in the current density range of from 10 to 10 amperes per square centimeter, the alpha is high and is substantially greater than 10 and relatively constant. For current densities progressively greater than 10 amperes per square centimeter, the alpha progressively decreases. For current densities progressively less than 10' amperes per square centimeter, the alpha also progressively decreases.

As the metal oxide varistor material is a ceramic material, the surfaces thereof may be metallized for facilitating electrical connections'thereto in a manner similar to the manner in which other ceramic materials are metallized. For example, Silver Glass Frit, duPont No. 7713, made by the duPont Chemical Company of Wilmington, Delaware, may be used. Such material is applied as a slurry in a silk screening operation and fired at about 550C to provide a conductive coating on the surface. Other methods such as electroplating or metal spraying could be used as well.

The nonlinear characteristics of the material results from bulk phenomenon and is bi-directional. The response of the material to steep voltage wave fronts is very rapid. Accordingly, the voltage limiting effect of the material is practically instantaneous. Heat generation occurs throughout the body of material and does not occur in specific regions thereof as in semiconductor junction devices, for example. Accordingly, the material has good heat absorption capability as the conversion of electrical to thermal energy occurs throughout the material. The specific heat of the material is 0.12 calories per degree Centigrade per gram. Accordingly, on this account, as well, heat absorption capability of the material is advantageous as a surge absorption material.

The material, in addition to the desired electrical and thermal characteristics described above, has highly desirable mechanical properties. The material has a fine grain structure, may be readily machined to a smooth surface and formed into any desired shape having excellent compressive strength. The material is readily molded in the process of making it. Ac-

cordingly, any size or shape of material may be readily formed for the'purposes desired.

For the switch assembly of FIGS. 1, 2, 3; and 4, the spacing of the opposed surfaces of the wafers 34 and 36 for a metal oxide varistor material of particular voltage gradient versus current density characteristics is set to provide an appropriate low current drain through the wafers under normal operating voltages for the switch.

While the invention has been described in specific embodiments, it will be appreciated that modifications may be made by those skilled in the art and I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure as Letters Patent ofthe United States is:

1. A switch assembly comprising:

a pair of conductive contact support members, one

of said members being resilient, each of said members having a pair of major opposed surfaces, said members being closely spaced with said surfaces substantially parallel,

a pair of contact elements, each contact element conductively secured to a respective conductive support member,

an element of metal oxide varistor material having a pair of opposed surfaces, each of said conductive support means conductively mounted to and supported by a respective opposed surface of said element,

said conductive contact support members being aligned so that said contact elements engage and disengage in response to movement of said resilient conductive support member.

2. The combination of claim 1 in which said material has an alpha in excess of 10 in the current density range of 10' to 10 amperes per square centimeter.

3. The combination of claim 1 in WhlCh the opposed surfaces of said element in contact with said conductive support members are spaced to provide a standby current flow between said conductive members which is low when normal operating voltage appears across said conductive members and when voltages in excess of normal voltage progressively appear thereacross rapidly decreasing the impedance is presented by said element in accordance with the alpha of the material thereby limiting the variation in voltage between said conductive members.

4. The combination of claim 1 in which said other conductive support member is resilient.

5. The combination of claim 4 in which each of said conductive contact support members are elongated and in which the longitudinal axes of said members are substantially parallel.

6. A switch assembly comprising:

a plurality of elongated conductive contact support members, each having a pair of major opposed surfaces, said members being closely spaced with respect to one another with said surfaces substantially parallel, the longitudinal'axes of said members being substantially parallel,

a plurality of said members being resilient, a plurality of contact elements, each contact element conductively secured to a respective conductive support member, a plurality of wafers of metal oxide varistor material, each wafer having a pair of major opposed surfaces, said conductive members interleaved with wafers so that a major surface of each wafer conductively engages a respective opposed surface of a contact support member and means for bonding said wafers and said conductive support members together. 

1. A switch assembly comprising: a pair of conductive contact support members, one of said members being resilient, each of said members having a pair of major opposed surfaces, said members being closely spaced with said surfaces substantially parallel, a pair of contact elements, each contact element conductively secured to a respective conductive support member, an element of metal oxide varistor material having a pair of opposed surfaces, each of said conductive support means conductively mounted to and supported by a respective opposed surface of said element, said conductive contact support members being aligned so that said contact elements engage and disengage in response to movement of said resilient conductive support member.
 2. The combination of claim 1 in which said material has an alpha in excess of 10 in the current density range of 10 3 to 102 amperes per square centimeter.
 3. The combination of claim 1 in which the opposed surfaces of said element in contact with said conductive support members are spaced to provide a standby current flow between said conductive members which is low when normal operating voltage appears across said conductive members and when voltages in excess of normal voltage progressively appear thereacross rapidly decreasing the impedance is presented by said element in accordance with the alpha of the material thereby limiting the variation in voltage between said conductive members.
 4. The combination of claim 1 in which said other conductive support member is resilient.
 5. The combination of claim 4 in which each of said conductive contact support members are elongated and in which the longitudinal axes of said members are substantially parallel.
 6. A switch assembly comprising: a plurality of elongated conductive contact support members, each having a pair of major opposed surfaces, said members being closely spaced with respect to one another with said surfaces substantially parallel, the longitudinal axes of said members being substantially parallel, a plurality of said members being resilient, a plurality of contact elements, each contact element conductively secured to a respective conductive support member, a plurality of wafers of metal oxide varistor material, each wafer having a pair of major opposed surfaces, said conductive members interleaved with wafers so that a major surface of each wafer conductively engages a respective opposed surface of a contact support member and means for bonding said wafers and said conductive support members together. 